1. Field of the Invention
The invention relates to the field of refining smelts of alloys containing iron, nickel and/or cobalt in which the base metal tends to form a carbide.
2. Description of the Prior Art
The use of halogens, e.g. F, Cl, for purifying aluminum is well known. The processes taking place in a gaseous state by using chlorine are mainly based on the decomposition of monochloride, which is stable at high temperature, into trichloride and metallic aluminum (molten) at a low temperature. Some of the impurities are removed even in the evaporation-condensation treatment. Numerous inventions have been made for removing calcium and magnesium from the gas (AlCl.sub.3) or gas mixture. The term "iron triad" is used to refer to the three metals, iron, cobalt and nickel, because of their proximity in the periodic table of elements and their somewhat similar properties. The processes are based on the condensation of these metals under temperature conditions corresponding to the lowest possible vapor pressures of their chlorides, in which case the components condensating separate solid or molten or by dissolving in salt melts. Thermal fluoride treatment is also used as analogous to chloride treatment. A suitable process for removing magnesium is to treat a mixture of aluminum and magnesium fluoride at a high temperature, (1450.degree.-1600.degree. C), at which time aluminum monofluoride and magnesium vapors are produced. When the gas temperature is lowered slowly, magnesium-free aluminum melt and magnesium fluoride are produced as a result of a reaction inverse to the former. The other impurities (Fe, Ti, Si, etc.) remain in the residue after the evaporation. The purification of molten aluminum (Mg, Na, etc.) can be carried out by using a molten salt, in which case the impurities are halogenated with a halogen separated from the salt or with a separately added halogen. The impurity halides and the produced aluminum halides are dissolved in the molten salt (NaAlCl.sub.4, MgCl.sub.2, 2AlCl.sub.3, alkali chlorides, earth alkali chlorides, and fluorides).
The purification of impure molten magnesium can also be carried out with a salt melt. In this case the salt melt is usually at the bottom of the purification chamber or chambers, and the molten magnesium flows above the salt slag.
Chlorination is also used for purifying molten cement copper while protecting it with a molten salt (Na.sub.3 AlF.sub.6), in which case the impurities of the molten metal are evaporated.
The halogenation of hard-metal carbides can take place at a low temperature (1000.degree. C) by a total chlorination (Cl.sub.2 + CO.sub.2), in which case W, Ta, Nb, Ti, etc. can be separated as chlorides from the binding material and carbon. The process for removing impurities from carboniferous materials (Ti-containing) by chlorinating (Cl.sub.2 + air) them as chlorides obviously belongs to the same group.
In the previously known processes for halogenating ferrous alloys (Fe-W, Fe-Nb, Fe-Cr) the aim is mainly a total evaporation at a low temperature (FeCl.sub.3, TaCl.sub.5, NbCl.sub.5, WCl.sub.5, CrCl.sub.2) and a fractional distillation of the obtained products of evaporation (Nb-Ta separation, etc.). In addition to halogens, molten salts are also used as halogenating agents.
The object of the purification of ferrochromium and ferrous silicon, in both a molten and a solid stage, has often been mainly the removal of carbon, hydrogen, oxygen, and nitrogen, usually in a vacuum process. As a common process for melts we can mention the process in which a CaF.sub.2 + Na.sub.2 CO.sub.3 mixture is injected into a melt, and the produced NaF removes both the impurities and the gases from it. Chlorination has also been used for the removal of the impurities (Ca, Al) from ferrous silicon in a molten state, although the sulfide melt-slag separation (Ti, Al, etc.) is also being developed further.
It can be said in general concerning the processes, based on halogenation, for metals or metal alloys that there is a multitude of methods developed especially for light metals. The halogenation is carried out by using either elemental halogens (F, Cl) or single or double salts formed by them with alkali metals, earth alkali metals, aluminum, etc., or salt mixtures. A salt mixture is also often used for dissolving and recovering the impurity halide or the basic metal halide. The processes for refining both light and heavy metals often include the halogenation of the metal to be refined, together with its impurities, and a fractional distillation thereafter. The halogenation of ferrous alloys usually takes place in a molten state, and thereby at a low temperature. On the other hand, there are most likely very few actual processes for halogenating heavy metal melts.